The invention relates to a method for checking the tightness of a tank-venting system of a vehicle.
Methods for checking tightness are shown, for example, in German patent publications 196 36 431 or from 198 09 384.
In these methods, air is pumped into the tank-venting system with a pressure source. In this way, a pressure builds up in a tight tank-venting system. The increased pressure changes the operating characteristic variable of the pressure source, that is, for example, the electrical current requirement of the pump of the pressure source is increased. The measurement of the pump current thereby defines an index for the pressure in the tank. The pump current is measured at the start of the pumping operation and after the elapse of a pregiven time interval. An increase of the current is expected in a tight tank-venting system because of the pressure which builds up therein. A fault announcement xe2x80x9cleakagexe2x80x9d is outputted when the current increase drops below an expected pregivable amount.
A tightness check of this kind can be successful only when there is a tight tank-venting valve. For example, a tank-venting valve (jammed in the open position), which is disturbed in its operation, can lead to an untight tank system. The operation of the tanking valve can be checked in different ways. For example, the current, which is required for driving the tank-venting valve, can be detected and evaluated or tank-venting valves can be used whose switch positions can be read out.
Operational checks of the tank-venting valve of this kind require additional electrical lines and additional circuit complexity. These operational checks therefore not only require a technical complexity but especially also a disadvantageous cost.
In view of the above, it is an object of the invention to so improve a method for checking the tightness of a tank-venting system of a vehicle of the above kind so that a reliable check of the operability of the tank-venting valve is possible with the least amount of technical complexity.
The basic idea of the invention is to draw a conclusion during tightness checks based on the time-dependent trace of the operating variable. For this purpose, the time-dependent trace of the operating variable is detected during the tank measurement and a conclusion is drawn as to the operability of the tank-venting valve from this time-dependent trace. A significant advantage of this method is that no additional lines, circuits, or the like are required in order to check the operability of the tank-venting valve. Rather, the time-dependent trace of the operating variable is evaluated during the tank measurement for checking the operability of the tank-venting valve which anyway takes place.
A comparison to a reference measurement can take place as well as a comparison to a threshold which is derived from operating variables for controlling an internal combustion engine of the vehicle or is fixedly pregiven.
An advantageous embodiment of the method provides opening the tank-venting valve after the end of a tank measurement; detecting the time-dependent trace of the operating variable; and, drawing a conclusion as to the operability of the tank-venting valve when the operating characteristic variable drops by a pregiven value within a pregiven time. In this embodiment, a check can be made in a simple manner as to whether the tank-venting valve is jammed in the closed state. In this case, no leak would be detected but nonetheless a proper functioning of the tank-venting system would not be ensured.
In another embodiment wherein the tightness of the tank-venting valve is checked, it is provided that the time-dependent trace of the operating characteristic variable is detected during a tank measurement executed during idle of the internal combustion engine of the vehicle (idle measurement). Then, the time-dependent trace of the operating variable is detected in a tank measurement carried out for a switched off internal combustion engine of the vehicle (run-on measurement) and a conclusion is drawn as to a disturbance of the tank-venting valve when the time-dependent gradient of the operating characteristic variable for the idle measurement departs from the comparable time-dependent gradient of the operating characteristic variable during the run-on measurement by a pregiven value. In this way, a check of the tightness of the tank-venting valve is possible in the closed state. In this way, especially a very fine leak in the tank-venting can be distinguished from a very fine leak at another location in the tank-venting system.
As mentioned above, one or more of the following variables can be used as operating characteristic variables: the current take-up by the pressure source and/or the rpm of the pressure source and/or the voltage applied to the pressure source and/or the pressure generated by the pressure source.
Insofar as a reference leak is used, it is preferably switched in parallel to the tank-venting system.